In a typical gas-phase, fluidized bed olefin polymerization process, fine polymer particles are carried overhead from the reactor and are removed from the recycle gas stream using cyclone or centrifugal separators. The collected fines are drawn from the bottom of the separator using an ejector and from there are returned to the reactor. Such a system is described in U.S. Pat. No. 4,882,400; the entire content of which is hereby incorporated by reference.
Ejectors discussed in the '400 patent have a horizontal design. A typical horizontally oriented fines ejector is shown in FIG. 1. In a typical horizontal fines ejector 10, a motive gas stream 11 enters the ejector 10 horizontally as shown in FIG. 1. The motive gas stream 11 provides a motive force to draw fines and gas 12 from the separator (not shown) into the ejector 10 and conveys them through outlet 13 back to the reactor (not shown) into which they are subsequently re-injected.
In a continuous gas-phase, fluidized bed polyolefin polymerization process, it is common that gas loop piping and equipment would foul to the extent that a shutdown is periodically required for cleaning. Fouling is especially severe in the horizontal fines ejector 10 as the polymer powder-laden gas stream 12 from the fines separators (not shown) is forced to change direction, typically 90°, within the ejector 10. The powder and sometimes even sheets of polymer tend to build-up in area 14 and reduce the efficiency or even plug the ejector 10.
Fouling is so severe in the conventional-design ejector that cleaning to remove polymer build-up is required on average every two months, resulting in frequent plant shutdowns, excessive cleaning expenses, and unacceptable lost production. In extreme cases, heavy fouling of ejector internals will significantly impede ejector performance and has resulted in flow blockages and unplanned reactor shutdowns.
To remedy this fouling problem, attempts have been made to improve the ejector design. One such attempt is described in U.S. Patent Appl. Publ. No. US 2008/0021178; the entire content of which is hereby incorporated by reference. Instead of using a horizontally oriented fines ejector, US '178 devised a vertically oriented fines ejector. While the vertically oriented fines ejector of US '178 ameliorates the fouling problem, the vertically oriented fines ejector is still susceptible to fouling or clogging periodically.
This invention aims to solve the fouling or clogging problems associated with horizontal and vertical fines ejectors.